This invention relates to a method and apparatus for recording uterine or vaginal electrical activity. More specifically, the invention relates to a method and apparatus useful for determining the contractility of the uterus by recording spontaneous, mechanically or electrically stimulated, or drug-evoked electrical activity of the myometrium of the uterus from the abdominal or vaginal surface.
Presently there is no objective manner with which to evaluate the contractility of the uterus. This is true either in nonpregnant patients where hypercontractility is associated with dysmenorrhea or in pregnant patients where the uterus is sometimes active prior to term. Normally the uterus is quiescent in nonpregnant women and during most of pregnancy. However, at the end of pregnancy the myometrium undergoes a series of changes that lead to synchronous, rhythmic uterine contractions (labor). The diagnosis of labor is the most significant problem faced by obstetricians. In addition, preterm labor, which occurs in about 10% of pregnant patients, is difficult to diagnose. Frequently term or preterm labor require adjuvant therapy to either stimulate or inhibit contractility of the uterus.
Since there is some minor spontaneous uterine activity at all times during pregnancy, it is often not possible to distinguish between this physiological activity and term or preterm labor. The state of the cervix is commonly used as a predictor of labor. However, the softening of the cervix occurs relatively late in labor. In addition, labor and changes in the cervix can occur independently. Alternatively the frequency of contractions is used to diagnosis labor, sometimes recorded with a tocodynamometer. However, these methods give only crude subjective estimates of uterine contractility.
The uterus does not contract vigorously throughout most of pregnancy and this provides a tranquil environment for the growing fetus. At term the uterus normally begins to contract forcefully in a phasic manner (labor) to expel the fetus. Contractions of the uterus are directly proportional to the underlying electrical activity of the muscle. The frequency, duration and magnitude of a uterine contraction are directly proportional respectively to frequency of bursts of action potentials, the duration of a burst of action potentials, and the propagation (also referred to as conduction) of action potentials over the uterus and the recruitment of muscle cells. A similar situation exists in heart muscle although heart and uterine muscle are different with respect to structure and configuration of the action potentials. The action potentials are accompanied by the influx of calcium into the muscle cells to activate the contractile apparatus.
Thus, by recording uterine electrical activity one can assess the contractility of the myometrium. Similar technology is used to record cardiac electrical activity to determine the normal or abnormal function of the heart.
Many studies have previously recorded uterine myometrial electrical activity using electromyography (EMG) where electrodes are placed directly on the uterus. These studies show that the myometrium generates little electrical activity prior to labor but activity increases tremendously during labor reflecting the mechanical events. Studies of interest are demonstrated in publications by Csapo, Chapter 43, "Force of Labor," Principles and Practice of Obstetrics and Perinatology, Ed. L. Iffy and H. A. Kaminetzky Publishing, John Wiley and Sons 761-799, 1981; Garfield et al., "Control of Myometrial Contractility: Role and Regulation of Gap Junctions," Oxford Rev. Reprod. Biol. 10:436-490; 1988; Wolfs and Van Leeuwen, "Electromyography observations on the human uterus during labor," Acta Obstet. Gynecol. Scand. [Suppl.] 90:1-62, 1979; and more recently by Devedeux et al., "Uterine Electromyography: A Critical Review," Am J. Obstet. Gynecol, 169:1636-1653, 1993. One may measure and use uterine EMG activity by direct contact with the uterus to predict normal and abnormal uterine contractions. However, it is not practical to place electrodes directly on the uterus. To do this under the present level of understanding one must surgically implant electrodes on the uterine surface or introduce a catheter electrode through the vaginal canal and puncture the fetal membranes.
It would be desirable to record uterine EMG activity from the abdominal or vaginal surface. However, previous studies of electrical activity of the uterus recorded with electrodes placed on the abdominal surface have failed to record bursts of action potentials from the uterus and generally show no association of uterine electrical activity with contractility. Studies of interest are included in the above-noted publications by Wolfs and Van Leeuwen, and by Devedeux et al. Wolfs and Van Leeuwen summarized all studies prior to 1979 and concluded that "it has never been clearly shown that the potential fluctuations obtained by means of electrodes attached to the abdominal wall, do indeed represent the electrical activity of the uterus." (Page 7.) Similarly, Devedeux et al state that abdominal monitoring of uterine electrical activity "requires further investigation." (Page 1649.)
Part of the difficulty in interpretation of electrical activity recorded from the uterus lies in the fact many investigators, including Wolfs and Van Leeuwen and Devedeux et al. have failed to recognize that action potentials drive the uterus to contract. Action potentials are not responsible for contraction of some smooth muscle tissues such as airway muscle and some vascular muscles and therefore many researchers confound the uterus with other smooth muscle tissues. Thus, many of these studies have attempted to correlate electrical activity with mechanical contractions in order to show that electrical activity is responsible for contractions. However, no study has measured uterine and surface EMG simultaneously and correlated these to contractions. Furthermore, it is now clear (From publications by Marshall, "Regulation of Activity in Uterine Smooth Muscle," Physiol. Rev. 42:213-227, 1962; Csapo, Chapter 43, "Force of Labor," Principles and Practice of Obstetrics and Perinatology, Ed. by L. Iffy and H. A. Kaminetsky, John Wiley & Sons, 761-799, 1981; Garfield et al., "Control of Myometrial Contractility: Role and Regulation of Gap Junctions," Oxford Rev. Reprod. Biol, 10:436-490, 1988; Garfield, Chapter 3, "Role of cell-to-cell Coupling in Control of Myometrial Contractility and Labor," Control of Uterine Contractility, Ed. R. E. Garfield and T. Tabb, CRC Press, 39-81, 1994), that action potentials activate the uterus to contract and that by measuring uterine electrical activity one can indirectly estimate contractility.